Calcining of crushed gypsum comprises converting calcium sulfate dihydrate by heating into calcium sulfate hemihydrate, better known as stucco. Prior calcining apparatus and methods have taken various forms. Traditionally, the calcining of gypsum has occurred in a large kettle, having a thickened dome-shaped bottom, against which a gas-fired flame is directed, with the kettle and burner flame being enclosed in a suitable refractory structure. There is usually an associated hot pit into which the calcined material is fed. The kettle must withstand temperatures in the 2,000.degree.-2,400.degree. F. range, hence requiring expensive fire box steel plate on its domed bottom, which was typically 13/4 inches thick. U.S. Pat. No. 3,236,509 typifies this type construction. This approach had numerous disadvantages, such as the extreme waste of hot burner gases, and the associated refractory brick enclosure which, when repairs or kettle shut-down were needed, first required a lengthy cool-down period.
Other calcining kettles, of the general type described above, have included supplemental submerged combustion designs where exhaust gases from the gas-fired burners were discharged directly into the kettle contents. Here, the gas flame directly impinged against the material being calcined, and there was an increased possibility of creating so-called "dead burn" material, i.e., insoluble anhydrite. U.S. Pat. Nos. 4,176,157 and 4,238,238 typify that type approach. Additionally, other prior art calcining kettles, of the general type described above, included a series of cross tubes which passed generally horizontally completely through the kettle, allowing the hot gases within the refractory structure and surrounding the kettle to be supplementally directed through the tubes, and thus, through the kettle contents to further heat the same. U.S. Pat. Nos. 3,307,915 and 4,163,390 typify this type kettle construction. There have also been horizontally-aligned, rotary calcining structures; U.S. Pat. No. 3,871,829 typifies this type approach.
Besides the above kettle constructions which normally require expensive refractory structure, there have also been refractoryless kettles using the submerged combustion principle, including those having auxiliary draft tube structure encompassing the main burner tube, so as to reduce formation of dead-burned insoluble anhydrite. U.S. Pat. No. 4,626,199 typifies this type construction. Additionally, there are so-called refractoryless conical kettles with various types of submerged combustion heating systems, again with the attendant risk of creating non-uniform stucco and dead burn material. U.S. Pat. Nos. 4,629,419 and 4,744,961 typify such conical kettle constructions. More recent calcining kettle modifications have included so-called "boost" burner constructions, including electrical boost calrods, see U.S. Pat. No. 4,744,963, and gas-fired boost burner designs, both added as supplemental heaters to traditional refractory-type kettle constructions.
The present invention overcomes the disadvantages of the prior art calcining kettle constructions and calcining methods, in several respects. The present invention includes the method of staging the location of multiple immersion tube burner coils within a calcining kettle; forming the initial, i.e., lowest, burner assembly of a low profile design, to permit quick kettle start-up; initially charging the vessel with gypsum material to substantially cover the lower burner unit; energizing the lower burner unit to heat the initial fill of material; thereafter continuing to fill the kettle with successive additional loads of material to cover the successive higher-staged burner tube assemblies; and energizing the successive higher burner tube assemblies to establish a continuous cycle of ground gypsum material conveyed to and calcined in the kettle.
The present refractoryless calcining kettle utilizes a series of immersion tube burner assemblies, preferably formed in, but not limited to, horizontally-aligned, serpentine-like coils, with each burner tube assembly located within a specific calcining zone within the kettle. Each burner tube exhausts exteriorly of the kettle to a common flue stack. The kettle is constructed of steel material of sufficient thickness to withstand the calcining temperatures required, and is preferably flat-bottomed. Thus, the kettle of the present invention, and its associated housing structure, can be of less overall vertical dimensions, as no dome-shaped bottom construction or underlying burner apparatus is required. Further, no extra fire-box steel cladding is required for the kettle bottom (such as was previously required to withstand the highly elevated temperatures at which prior art kettles had to operate). Since all heating of the gypsum material occurs within the kettle, and since there is no external heat applied to the kettle, no separate refractory structure is required. Also, since no hot burner gases directly impinge on the gypsum material being calcined, but instead, the immersion tube burner gases exhaust from the burner tube coils exteriorly of the kettle through a flue stack, low capacity combustion air blowers can be utilized for the tube burners. This results in reduced initial capital expenditures.
Further, because the present invention uses a series of coil-shaped immersion tube burner assemblies which are mounted substantially within the calcining kettle, essentially all the burner-generated heat is transferred to the kettle contents, with attendant increased heat transfer efficiency.
Further yet, since additional burners and heat exchangers, i.e., burner tube coils, can be installed within the presently disclosed kettle vessel, even more heat transfer area can be installed in the same size kettle resulting in yet higher tonnage output capacity for the kettle. Thus, a greater tonnage output, for the same size kettle, can be achieved with the present kettle and burner design than with any known prior art design. As modifications to the present invention, additional flue gas recycling and aeration equipment can be installed adjacent the interior kettle bottom, to help with further heating and circulating of the kettle's contents.
An alternate embodiment comprises, instead of a series of immersion tube burner coils, a single burner tube coil mounted interiorly of a kettle, and still exhausting exteriorly of the kettle, to provide the total heat transfer for the kettle's contents. Such a single burner tube coil kettle is suitable for use in specific applications where a smaller capacity kettle is sufficient.
It is a primary object of the present invention to provide a calcining kettle utilizing at least one, interiorly-disposed immersion tube burner coil, which coil exhausts to the kettle's exterior, and where the kettle's contents are substantially entirely heated by that burner tube coil or coils.
It is another object of the present invention to provide a calcining kettle having multiple internally-disposed immersion tube burner assemblies, each preferably formed as a horizontally-aligned coil and operating in a separate calcining zone within the kettle.
It is a further object of the present invention to utilize as the lowest or initial immersion tube burner assembly one which is of a low profile, serpentine-like coil design, so as to permit quick initial start-up cycle for the calcining kettle, as well as to require a lower horsepower for the agitating drive device during cycle start-up.
It is a further object to provide a calcining kettle having no separate refractory enclosure structure, and where the kettle has reduced material requirements due to the substantially reduced temperatures at which it operates.
It is a yet further object of the present invention to have a calcining kettle that does not require a heavy duty, domed-bottom construction and eliminates any burner construction beneath the kettle, such that kettle-bottom aeration can be utilized.
It is a still further object to provide a calcining kettle construction which permits a substantially increased tonnage output of calcined material, for the same size kettle.
It is a still further object to provide a kettle design substantially teardrop-shaped in cross section which permits an elongated initial straight section for the respective immersion burner tube assemblies. Alternatively, the kettle design could instead be of a circular cross section design, with exteriorly-mounted burner boxes.